1. Field of the Invention
The present invention relates to a radio receiver and more specifically to a radio receiver having the diversity receiving function.
2. Description of the Related Art
In the current radio communication technology, various communication systems such as time division communication system, code division communication system have been introduced. These communication systems have a radio transmitter and a radio receiver as illustrated in FIG. 1.
In FIG. 1, reference numeral 1 designates a radio transmitter, while 7 designates a radio receiver. The radio receiver i comprises a signal processing section 2, a filter section 3, a D/A converter 4, a radio signal transmitting section 5, and an antenna 6. Meanwhile, the radio receiver 7 comprises two different antennas 8, 9 for diversity reception, radio signal receiving sections 10, 11, A/D converters 12, 13 corresponding to the respective antennas, filter sections 14, 15 corresponding to the respective antennas, a diversity combining section 16 for diversity combining outputs of the filter sections 14, 15 and a signal processing section 17 for processing the combined signal.
Next, operations of respective sections for transmitting a radio signal to the radio receiver 7 from the radio transmitter 1 will be briefly described. First, in the radio transmitter 1, the transmitting data to be transmitted from the signal processing section 2 is outputted as a digital signal and is then inputted to the filter section 3. As the filter section 3, a route Nyquist filter, for example, can be used. The filter section 3 suppresses inter-symbol interference of the input signal. An output signal from the filter section 3 is inputted then to the D/A converter 4, converted to the analog signal, and inputted to the radio transmitting section 5. The signal outputted from the signal processing section 2 and inputted to the radio transmitting section 5 is usually called a baseband signal.
The radio transmitting section 5 performs modulation process conforming to the predetermined modulation method (for example, orthogonal modulation or the like) on the basis of the input analog baseband signal and moreover transmits the radio signal from the antenna 6 after frequency conversion of the modulated signal to the radio frequency signal. When the radio transmitter 1 corresponds to the communication system of the code division system, the spread signal by the spread code is outputted from the signal processing section 2.
The radio signal transmitted from the radio transmitter 1 is received by two antennas 8, 9 installed at the radio receiver 7. Usually, these antennas are provided as the separated antennas for diversity reception. Accordingly, similar radio signals transmitted from the radio antenna 6 are respectively received with the antennas 8, 9 through different transmission routes.
The signals received by the antennas 8, 9 are demodulated in the radio signal receiving sections 10, 11 depending on the modulation method in the radio signal transmitting section 5 of the radio transmitter 1. Therefore, the baseband signals as the analog signal obtained by demodulation are outputted to the A/D converters 12, 13, respectively. When radio transmitter 1 introduces the orthogonal modulation method, the radio receiver 7 performs the orthogonal detection and provides the so-called parallel outputs of the I and Q elements. However, the parallel output is illustrated as a single output of transmitter 1 to simplify the description (radio receiving section 10 to signal processing section 17, radio receiving section 11 to signal processing section 17).
The A/D converters 12, 13 convert the received analog signal to the digital signal and then output the digital signal to the filter sections 14, 15, respectively.
As each of the filter sections 14, 15, a route Nyquist filter or the like is used as in the case of the radio transmitter 1 and applies the digital signal suppressing the inter-symbol interference to the diversity combinig section 16. The signal receiving system up to the diversity combining section 16 from the side of antenna 8 is called a first signal receiving system, while the signal receiving system up to the diversity combining section 16 from the side of antenna 9 is called a second signal receiving system, for discrimination of the signal receiving systems.
The diversity combining section 16 combines (for example, by addition) the received digital signals obtained through the first and second signal receiving systems and then applies the signal having suppressed noise through the diversity gain to the signal processing section 17.
The signal processing section 17 extracts a control signal and an information signal or the like included in the input signal to provide the output depending on the information signal to an output section (not illustrated) or to control each section in the radio receiver 7 with reference to the control signal.
The Japanese Published Unexamined Patent Application No. 1994-37685 (JP) discloses a general constitution of the diversity receiving apparatus.
A radio receiver provided with the ordinary diversity receiving function has the elements as described above and operates on the basis of such elements.
However, the above described conventional radio receiver has the following problems.
The radio signal receiving sections 10, 11 of the radio receiver 7 outputs, to the A/D converter, the signal having completed receiving processes such as suppression of the signal not in the specified frequency band with a filter such as BPF, frequency conversion, orthogonal detection or the like and the A/D converters 12, 13 respectively convert the analog signal of the radio signal receiving sections 10, 11 into the digital signal. Here, the A/D converters 12, 13 sample, for conversion into the digital signal, the analog signal with reference to the same timing using an output signal from a common oscillator (not shown in FIG. 1). Here, as the sampling frequency, the frequency, for example, which is higher by two or more times than the signal bandwidth of the baseband signal applied from the radio signal receiving section 10, is used.
As the optimum timing for the sampling, the Nyquist point receiving less amount of influence of other codes (time corresponding to 0.5 symbol in FIG. 2, namely the point indicated with an arrow mark) may be suggested. However, when it is assumed that the frequency which is equal to two times the signal bandwidth is used as the sampling frequency, the ideal A/D conversion can be realized when the sampling timing matches the Nyquist point. However, if the sampling timing is deviated from the Nyquist point, the signal is intensively influenced by inter-symbol interference and thereby a signal to noise ratio of the received signal is deteriorated to a large extent. Here, one symbol is the signal corresponding to one modulation period and also to one data (for example, the data “0” or “1”) when the modulation is performed with one data period. In this case, when the data is spread with the spread code and modulation is performed within the period corresponding to one chip after the spread, one symbol corresponds to the signal corresponding to one chip.
As a measure for improving quality of the received signal deteriorated due to deviation of such sampling timing, it is possible to perform the sampling with the timing nearer to the timing of the Nyquist point by realizing the over-sampling with the sampling frequency which is higher by two or more times than the signal bandwidth. However, it is generally known that as the sampling frequency is further raised, the circuit configuration of the radio receiver becomes more complicated and power consumption is increased, making it more difficult to realize reduction in size of the radio receiver and lower power consumption by the radio receiver.
Moreover, particularly when the QAM system of 16-level is introduced and the code division system allowing higher multiplexing degree of codes is employed as a radio receiver or when a radio receiver having only the simplified error correction capability is used, a significant problem of such sampling timing in the radio receiver occurs and thereby deterioration in quality of a receiving signal resulting from the deterioration in the signal-to-noise ratio is extremely increased.